Telephone-exchange system and apparatus therefor.



A. M. BULLARD.

. TELEPHONE EXCHANGE SYSTEM AND APPARATUS THEREFOR.

APPLICATION FILED JULY 17,1906.

1 ,21 9,700. A Patented Mar. 20, 1917.

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TELEPHONE EXCHANGE SYSTEM AND APPARATUS THEREFOR- APPLICATION FILED JULY 11,1906. 1,21 9,700. Patenped Mar. 20, 1917.

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OM QvZMM M A. M. BULLARD.

TELEPHONE EXCHANGE SYSTEM AND APPARATUS THEREFOR. APPLICATION FILED JULY 11.1906.

1,21 9,700. Patented Mar. 20, 1917.

8 SHEETS-SHEET 3.

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A. M. BULLARD.

TELEPHONE EXCHANGE SYSTEM AND APPARATUS THEREFOR- APPLICATION FILED JULY 17.1906.

1 ,21 9,700. w Patented Mar. 20, 1917.

8 SHEETS-SHEET 4.

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A. M. BULLARD.

TELEPHONE EXCHANGE SYSTEM AND APPARATUS THEREFOR.

APPLICATION FILED JULY 17,1906.

A. M. BULLARD.

TELEPHONE EXCHANGE SYSTEM AND APPARATUS THEREFOR.

APPLICATION FILED JULY 17,1906. 1,219,700. Patented Mar. 20, 1917.

8 SHEETS-SHEET 6.

7 ATTORNEYS A. M. BULLARD.

TELEPHONE EXCHANGE SYSTEM AND APPARATUS THEREFOR.

APPLICATION FILED JULY 17,1906.

1,219,700. Patented Mar. 20, 1917.

8 SHEETS-SHEET 7- WQIjESSES: INVENTYJR BY A # 6 ATTORNEYS m: wcwms psrsns ca. PHOTU-LITNQ. WASHlNcmN, u. c.

A. M. BULLARD.

TELEPHONE EXCHANGE SYSTEM AND APPARATUS THEREFOR.

APPLICATION FILED JULY 17,1906.

Patented Mar. 20, 1917.

8 SHEETSSHEET 8.

NINGTDN. n c

ALBERT M. BULLARD, OF NEW YORK, N. I

., ASSIGNOR, BY MESNE ASSIGNMENTS, TO

WESTERN- ELECTRIC COMPANY, INCORPORATED, A CGRPOB-ATION OF NEW YORK.

TELEPI-IONE-EXCI-IANGE SYSTEM AND APPARATUS THEREFOR.

Application filed July 17. 1906.

To all whom it may concern:

Be it known that l, ilkLBER T M. BULLAun, residing at New York, in the county or slew York and State of New York, have invented certain new and useful Improvements in Telephone-Exchange Systems and Apparatus Therefor, of which the following is av specification.

My invention relates to an automatic switching system for telephone exchanges, and its object is to provide an improved organization of circuits and apparatus where by a line to which connection is desired may be selected with speed and accuracy; the apparatus required being comparatively simple and inexpensive, and acapied to secure the greatest reliability and efiiciency of operation.

Another feature of my invention, which has reference particularly to a full automatic system in which the selective connecting apparatus is controlled exclusively from the subscribers stations, involves the use of a comparatively few automatic connecting devices, which may be seized for use by any one of a number of telephone lines. Mechanism. is provided for constantly causing the idle connecting devices to be brought successively into operative relation with the various lines of the exchange. The connecting devices are thus made available, one

after another, for use by any line which chances to be calling, so that the calls are evenly distributed among them, the interval during which a call is answered b ng reduced to a minimum, and there l. no possibility of two devices taking up the same call.

Another feature contemplated by my invention is that of terminating the lines ontering the exchange upon terminals so related to the interconnecting members of the switches which serve to establish communication between the various lines, that a le set of terminals individual to a pars ticular line is accessible to all of the connecting (.evices or automatic switche By providing terminals of this character it not only becomes unnecessary to provide separate terminals for each. line upon each switch and connect these terminals together by multiple wiring; but, by so arranging the single group of terminals of the line adapted to be engaged by all. oi the connecting devices or members that the group will be sligntly dis- Specification of Letters Patent.

Patented Mar. 20, 1917.

Serial No. 326,531.

placed in position when connection is completed with it by any of the connecting devices, it becomes possible to move the terminals bodily out of the possibility of engagement with any other of the connecting membt-irs thus preventing the establishment of connection between the engaged line and any other 01" the connecting members which might otherwise have access to it. The movement or displacement of the group of terminals of each line common to all of the devices may also be utilized, as is done in the system of my invention, to so change the position. of a part associated with the group of terminals with relation to the other coiinecting devices that a busy signal automatically given when another of the connecting devices is moved into position to make connection with the engaged line.

By dispensing with connecting devices permanently associated with and individual to each line, and substituting a lesser number of connecting devices common to a group of lines and made successively available to each line in the group, the circuits of the lines are left normally open at both ends, and the place of the customarily employed line relay normally associated with and individual to each line is taken by magnets associated with the common connecting devices or switches. These magnets control the associated connecting devices or switches in finding and making connection with a calling line, and in selecting and making connection with the line with which the calling line oesires to communicate; and, by being included together with the common source ot current in a brf dge of the connecting circuit, they are also made to serve as retardation coils to regulate the supply of energizing current to the connected telephone lines and prevent the shunting of the voice currents.

My invention also involves a number of other features, and novel apparatus which may be advantageously employed in practising the system oi? my invention, as will be particularly described in the tollowing por tions of the specification.

Before entering into a detailed. description of the apparatus and circuits employed in the system of my invention, in order that the system in its entirety may be more clearly comprehended I shall briefly outline its salient features as represented in the particular embodiment employed in the present in,- stance. At the center of a cylindrical group of line terminals, each of which consists of a. number of parallel metallic rods vertically arranged, is supported a rotating shaft hearing a number of pairs of switch arms or connecting members loosely mounted thereon, but normally locked to the shaft by electromagnetically controlled keys. At the outer extremity of each pair of arms are contact springs adapted to be thrust forward, by electromagnets forming a part of said arms, into engagement with the vertical rods. Each pair of arms constitutes a complete connecting device: corresponding, in effect, to a pair of connecting cords in a manual switch board. The arms of a pair are normally so locked to the shaft. each by its own key, that they lie in the same vertical plane and are revolved as one. One of the arms, the upper one in the present arrangement, bears a flexible spring which tests with awiping contact each idle line in turn until. it finds one wishing to make a call. That the line is calling is indicated by an altered electrical condition of its terminal brought about by the act of taking the telephone from the hook at the substation. To this altered electrical condition the magnets of the connecting arms are responsive, and when the wiping spring makes contact with the terminal the magnets operate to disengage both arms from the shaft and to cause one of them to make contact with the terminals of the calling line. The vertical line terminal rods are so mounted that when they are pressed upon by the connecting springs of the arm they are turned slightly upon a central supporting rod, and are thus withdrawn from the path of the test springs of all the other pairs of arms. Furthermore, this movement brings into prominence a terminal connected with the busy tone apparatus, which protects the line from being intruded upon.

The process of selecting the line with which connection is desired is effected by manually moving a device at the substation through a. predetermined arc, corresponding with the degree of separation between the terminals of the calling and the terminals of the called line on the central ollice switch. This deenergizes the electromagnet of the lower arm and allows the latter to reengage the rotating shaft. whereupon it resumes its revolution, leaving the upper arm behind. In thus causing the arms to separate. the line current is changed to intermittent current by a device common to both arms. This current causes a reverse movement of the substation device until the latter returns to its normal position, when it causes the mag nets of the lower arm again to become energized. This stops the lower arm opposite the line terminals of the desired subscriber, and causes the arm to make contact therewith, provided the desired line is not already engaged. If it is engaged, the protruding busy tone terminal associated with the line prevents the magnet of the arm from operating to bring the connecting springs of the arm into contact with the line, and the busy tone signal is transmitted to the calling subscriber. The calling or selecting arm of the pair is arranged to reverse the connection of the common battery with the line of the called subscriber in making contact with the line terminals, and thus to cause a vibrating bell at the called station to ring until the subscriber answers.

lVhen the subscribers hang up, the mag nets of both arms are deenergized, and the keys of each are ready to rei ngagc the slot in the shaft when next it passes beneath. them. Pending this rei ngagement. each arm is held stationary by a. fork which engages a liXed point in the cylinder of line rods until the key drops into place. The arrangement is such that a pair of connecting arms cannot be appropriai'cd by a calling line until, the arms, which have been moved apart in the act of establishing connection between two lines, have re-unit-ed.

I will describe my invention more particularly by reference to the accompanying drawings, wherein Figure 1 is a front elevation, with-a portion of the face broken away, of the substation interrupter which. I prefer to employ in the system of my invention; Fig. 2 is a side elevation of the interrupter, with a part of the case broken away to show the arrangement of the apparatus within; Fig. 3 is a front elevation of the automatic switchboard of my invention, with most of the line terminal rods broken away to show the pairs of connecting arms mounted in the interior; +1; is a top plan view of the automatic switchboard illustrated in Fig. 3; Fig. 5 is a side elevation of a single pair of connecting arms with associated groups of line terminals; Fig. 6 is a plan view of a pair of connecting arms; Fig. 7 is a plan view of the selector arm, or lower one of the pair of connecting arms; Fig. 8 is a plan view of the ends of the sclector arm with its connecting springs at one end and its anchoring fork at the other, thrust into cngagen'ient with the corresponding groups of line terminals; Fig. 9 is an elevation of the anchoring ends of a pair of connecting arms; Fig. 10- is a i end elevation of the connecting ends of a pair of arms; Fig. 11 is a plan view of the hub of the finder arm, or upper arm of the pair, with portions broken away; Fig. 12 is a like view of the hub of the lower or selector arm; Fig. 13 is a front elevation of the hubs of a pair of arms partially in section in the central vertical plane of the arms; Fig. it is a rear elevation of the hubs of a pair of arms,looking in the opposite direction win from the view shown in Fig. 5; Fig. 15 is a diagrammatic representation of two subscribers lines extending from their respective substations to the automatic switchboard at the central office, all being equipped and arranged in accordance with the system of my invention; Fig. 16 illustrates the wiring of a pair of connecting arms employed in the system of my invention; Fig. 17 is a skeleton diagram illustrating certain elements of the specific system shown, which are involved in controlling the operation of a. selector or traveler by controller located at a distant point, which controller is in turn operated (to an extent which may be predetermined) by current impulses transmitted back to said controller by the selector or traveler in its advance.

Referring to Figs. 1 and 2, which represent the substation interrupter which I have devised. and find convenient to employ in my automatic telephone system, and which is desi nated by the reference character a, the dei re comprises an inclosing box or case (11, n1 hin which is mounted thi apparatus which constitutes the interrupter. This apparatus consists of a spur gear a mounted upon a shaft (2? which has hearings in a frame a? as secured to the front of the inclosing case c The shaft (0 projects through the front of the case and carries a pointer arm a, the extremity of which is adapted to be moved about the dial (616, in the present instance, in fifty steps, the steps being indicated by the numerals 1 through 50. The spur gear a? engages a pinion a mounted upon an arbor having its bearings in the frame a and upon the arbor a star wheel a? is mounted. The star wheel is adapted to be rotated by the escapement 1ever a which extends from and is moved by the armature a of an electromagnet a suitably supported upon the frame. The armature a is normally held in a retracted position away from the poles of the electromagnet by the spring a. The pointer arm at is adapted to be moved by hand, together with the spur gear and star wheel associated with it through the shaft a and may be provided with a knob or handle a for that purpose. The angle of the teeth of the star wheel a is sufficiently obtuse to allow the latter, and all parts associated with it, to be freely rotated in either direction between the teeth of the escapement lever a by means of the pointer arm. The star wheel and the cooperating escapement lever are so related that when the magnet is caused to move the armature a back and forth, the pointer arm is rotated in a contraclockwise direction.

Upon the shaft a is mounted a head or commutator a. This commutator has a groove or depression cut in its periphery in which one of the members of the switch a 5 is normally adapted to lie. l l hen the pointer arm and its associated parts are rotated from the position which they normally occupy, the switch member rides out of the depression, and is thrust into contact with the other member of the switch a, thus completing the circuit between them. The hub or commutator al is loosely mounted upon the shaft (0 but is normally held from rotating thereon by the set screw (i When desired, the an ular relation of the groove in the commutator to the pointer arm may be altered by loosening the set screw and turning the commutator upon the shaft (6 In using the interrupting mechanism described in the foregoing, the pointer arm normally rests opposite the number on the dial which represents the station with which the interrupter is associated. The commutator is secured by means of the set screw 5 in such angular relation to the pointer arm that the associated member of the switch a rests in the groove or depression. To set the device for calling any particular line in the exchange with which connection may be desired, the pointer arm is manually rotated by means of the handle a so that it rests opposite the number on the dial, which represents that line. lVhen thus moved the commutator has been rotated with relation to the switch (615 so that the latter is caused to close its circuit, In the actuation of the connecting mechanism at the central. office the circuit through the electro magnet a of the interrupter, which circuit also includes the switch a, is intermittently completed and broken. The impulses of current thus produced actuate the armature a of the magnet, and cause the escapement lever a to return the pointer arm in a contra-clockwise direction toward its normal position, the pointer arm being returned one step for each impulse of current. l i hen as many impulses have'passcd as the number of steps the arm has been moved from its home or normal position, the depression of the commutator comes under the cooperating member of the switch a, and the switch operates to break the circuit and stop further movement.

Associated with the interrupter, and cooperating with it in the control of the circuit, is an olectromagnet a suitably mounted beneath the interrupting mechanism above described, and in the same inclosing case. The armature a of this electromagnet is carried upon the plunger (Z of a push lntton a, w iioh is adapted, through the medium of the plunger, to operate the switch (6 consisting of three contact springs, one of which is arranged normally to make contact with another, and to interrupt that contact and complete connection with the third spring when the plunger is moved by the de )ression of the push button a.

When the push button is moved to change the circuit through the switch a, the armature e is brought into contact with the poles of the electromagnet'a and the energizing current which flows through the magnet on account of the change in the circuit controlled by the switch a causes the ar mature to be attracted, and the key to be held in its operated position, until the circuit is interrupted at another point.

I will now describe the automatic switch mechanism illustrated in the drawing, Fig. 3 to F 14, inclusive. The switching mechanism in its entirety is designated R and comprises a general framework built, in the present instance, in halves 7, r each half comprising semi-circular segmental top and bottom plates ri and 7", r respectively, held together by the supporting rods 9". The halves *1 and r of the framework are supported upon a. base T and are fulcrumcd to swing upon supporting shafts 1' 1", which are firmly secured in the base 1' and extend up through one end of each of the halves of the framework. At the other extremity of each of the halves 7', 7' are like rods 9, T, which, however, do not extend into the base T, but merely serve to strengthen the framework. The extremities of the upper segments 7, r are united bv the bracing pieces 7- and 9". Through the extremities of the halves 1", T of the framework opposite the points where they are hinged or fulcrumed, locking pins 1', r extend, thus fastening the halves of the framework securely together.

Mounted in the halves r, r of the framework, and pivoted at both ends in the top and bottom segmental portions 1 ,1, 1*, 9', respectively, are the groups of line terminals (Z. Each group comprises a number of metallic rods insulatedly mounted at the top and bottom in the plates (Z which carry the pivots, and are held ri idly by the collars d, called ground rings". The construction and arrangement of the groups of line terminals may more readily be seen in Figs. 5, 6 and 7 of the drawing. Each group, in the present instance, comprises four rods (Z (F, d and r of which (Z and (Z are terminals to which the two limbs of the associated telephone line. extend, while (Z is grounded or connected to earth, while (Z is connected with a source of busy-tone current. The ground rings (Z are metallically connected with the ground rod (Z of each group, but are insulated from the other three rods. Mounted upon the line terminal rod (Z of each group, but insulated therefrom, are a number of busy-tone rings (Z which are metallically connected by means of a part (Z extending from the ring (shown in Fig. 7 with the busy-tone rod (Z of the associated group. The ground rings (F associated with each group are equal in number to the pairs of connecting arms with which the switch mechanism is provided, and are so located as to be directly opposite and adapted to be engaged by a contact spring (2 *arried by the upper arm of each pair. The busy-tone rings d" are also equal in number to the pairs of connecting arms with which the. switch is provided, and are so located as to be engaged, under certain conditions, by a contact spring f, carried by the lower arm of each pair. As has hcreto tore been stated, each group (Z of terminals is pivotally mounted between the upper and lower segmental plates of the frame, the bearing for the upper pivot being preferably formed in the end of a screw provided with a. lock nut (.Z by means of which the hearing may be adjusted. As constructed in the present instance, the ground rod (Z of each group coincides with the longitudinal pivotal axis oi? the group. Each group is provided with a ret'ractile spring (Z extending between the lower pivot plate oi the group and a stationary butment, and which serves to yicldingly hold the group in a position with its line terminal (Z :ulranced toward the center of the switch and its line terminals (Z with the associated busy-tone rings (Z retracted. The degree to which. the group of terminals may be rotated by the spring (Z is limited by the recovery stop (i associated with each group of terminals.

lVithin the cylindrical cage constituted by the groups of terminals (Z (I have illustrated fifty such groups in the present instance), a number 01 pairs of connecting arms E, F, are arranged to rotate. In the present instance, I have illustrated six pairs of arms, each pair being loosely mounted upon a shaft 1' rotatably supported along the longi tudiual axis of the switch, and journaled in the bearings 1 r in the bottom and top of the frame. The portion of the shaft W that carries the arms is insulated from the frame by means of short sections of insulating material. r and 1* let into the shaft at its top and bottom. The shaft is rotated through the medium of the beveled gears r, r", the former of which is carried upon the shaft T and the latter upon the countershai't r, j ournaled in the base 1- and provided at its outer end with the pulley 1 by means of which power may be communicated to the shaft to cause its rotation.

As heretofore stated, each pair of arms is of the drawings. Each arm, as constructed in the present instance, comprises two tubular electro-magnets secured to the hub of the arm, and extending radially therefrom in opposite directions. The upper or finder arm E of the pair comprises the tubular magnets 7t and g, which extend from the hub g loosely mounted upon the central rotating shaft; while the lower or selector arm F comprises the tubular magnets and Z extending radially from the hub Z The magnets it and 7c of the arms E and F, respectively, are adapted to actuate devices which control the connection of their respective arms with the rotating shaft, and other devices which anchor or clamp their respective arms to fixed points in the surrounding cylinder of terminal groups. The magnets g and Z of the arms E and F, respectively, serve to actuate switch parts which complete connection between the circuits of their respective arms and the groups of line terminals which surround them.

l'teferring more particularly to the upper or finder arm E of the pair, the magnet 72 which extends from one side of the hub and constitutes, with its associated parts, one-half of the arm, comprises a tubular shell if of iron within which extends an iron core h (see Figs. ll and 1.3), surrounded by a winding 72.1. At the inner end of the magnet is an armature 72. which is so constructed and related to the end of the pole-piece it as to permit of a considerable range of movement. From the armature 7L2 extends a key in, which is adapted to engage a slot in the collar 1 fixed to the shaft 7, the arrangement being such that when the armature is retracted the key engages the slot in the collar thus connecting the arm E with the rotating shaft. Another portion of the armature 72, adapted to engage and operate (through the medium, in the present instance, of a lateral extension of the armature bearing an adjustment screw fi l' a switch it, the members of which are normally held in contact by the engagement of the armature with one of them, but which are adapted to separate when the armature is attracted to the magnet. The extent of separation of the switch contacts is limited by the stop it.

The armature 72 of the magnet is carried upon the end of two side rods 72 which ha Ye. bearings at both ends in enlargements of the ends of the magnet shell 715" so as to be slidably movable therein, and which carry at their forward extremity, beyond the outer end of the magnet 7a, a yoke 71. This yoke, as illustrated, is insulated from the rods h and carries upon an arbor h extending between its extremities a loose sleeve to which is secured the fork or anchoring device h The upper end of the fork is pronged or bifurcated so as to emterminals opposite which it may come to rest. The lower end of the fork or anchor it is journaled upon and insulated from the end of a rod it, the other end of iflilCil is journaled upon and insulated from an extension carried upon the outer end of the magnet shell. if. Encircling the two side rods it are helical compression springs 71?, interposed between the enlargement at the forward end of the magnet through which the rods pass, and sleeves secured to the rods. These springs normally serve to keep the armature h in its retracted. position, the key 72/ in engagement with the slot in the fixed collar 2', and the fork or anchor it out of engagement with the ground or pivoted rod of the encircling groups of terminals. When the magnet 72 energized, the key it" is withdrawn and the fork it hrust forward to anchor the arm against turther rotation.

The yoke which completes the magnetic circuit between the core It and the shell it of the magnet 71 at its forward or outer end, is in. the form of an armature 71" which is normally retracted, and which, when the magnet is energized, is actuated to complete the circuit between the contact rods 7:." by causinga contact plate it carried at its upper end to engage the ends of the contact rods, and thus close the circuit in which they are included.

Extending from the hub of the upper or finder arm E, and diametrically opposite the magnet 75, is the connecting magnet g. This magnet is also of the iron-clad tubular type, and comprises an iron shell g within which is an iron core 5/ surrounded by a winding 9 At the inner end of the ma g net, or the end next the hub, is a armature so constructed and related to the pole of the magnet that it is capable of a considerable range of movement. The armature is carried at the ends of the two side rods which have hearings in the enlarged ends of the shell of the magnet ,7, so that they can slide longitudinally therein. Upon the outer ends of the side rods and in front of the forward end of the magnet g, is mounted an insulating block in which are secured a number of contact springs that are adapted to make connection with the terminal rods of the groups which encircle the arms. These comprise, in the present instance, the two line contact springs c (1", each of which comprises two metallic strips separated by a tongue of insulating material and having their outer ends turned outwardly to form afork adapted to embrace the terminal rod, and a test spring 0 which projects farther than the line springs c and c and is adapted, when the block 9 is in its retracted position, to make a wiping contact with tho advancedterminal rod (Z of: each group of terminals. The armature g and the contact springs mounted upon the block which it controls, are normally held in their retracted position by helical compression springs which encircle the side rods 5/, and are interposed between fixed collars carried. upon said rods and the forward enlarged head of the magnet shell f thri'iugh which the rods pass. Upon the encrgization of the magnet, the armature is attracted, and the springs carried upon the block 1 are thrust forward, the cngan' of rho contact spring with the advanced teru'iinal rod (F of the opposite group of line terminals causing the group to turn upon its lmigitudinal axis, and thereby oring tl 0 terminal rod (Z of the group forward and into engagement with c that has advanced to meet it. Connection between the contact springs carried on the block and other parts of the apparatus is completed through the medium of the connection rods 5 Another insulating block is carried upon the lower side of the outer enlarged head of the magnet shell 9'', and upon it are mounted a contact spring a, which is a (lapted to engage the corresponding ground ring (Z of each group of terminals as the arm swings past it, and a contact spring 6 which extends downwardly, and is adapted to en gage a cooperating contact part carried upon the lower or selector arm The lower or selector arm. F in its construction is, in general, similar to the upper or finder arm It comprises a movement controlling magnet and a connecting magnet Z extending radially and in opposite directions from a hub Z loosely mounted upon a rotating shaft r As in the movement controlling magnet of the finder arm E, the magnet Z: of the lower or Selector arm comprises a tubular iron shell h, within which extends an iron core 1: surroiunled by a winding 70 The armature 7,: of the magnet, adapted to have a considerable range of movement, is carried upon the ends of the two side rods 70', which are slidably supported in the enlarged heads at the ends of the magnet shell 6 are provided with the encircling helical compression springs to keep the armature and its associated parts normally retracted, and carry upon their outer ends, and in front of the forward end of the magnet 71. a yoke c which serves to operate the fork or anchor 70. The fork or anchor is fulcrumed upon the arbor or pivot rod 0 of the yoke, and has its lower extremity connected with the forward end of the magnet shell by means of the connecting rod Z2 insulating material being interposed at suitable points to effect the insulation of the fork or anchor from the metallic portions of the arm. As described in the contact spring.

connection with the movement controlling magnet of the upper arm E, the magnet 7", through the medium of its armature, controls a. key it which, in the retracted position of the armature, engages a slot in the collar 1 fixed to the shaft, and also serves normally to keep the switch L with its contact parts together, the switch being permitted to separate its contacts when the magnet is energized and the armatln'e attracted.

As described in connection with the arm it, the magnetic circuit between the core Z' and the shell L of magnet Z: is cou'iplctcd by an armature a" which is normally retracted, and which is actuated, when the magnet is energized, to complete the circuit between the contact rods Z1" by causing the contact plate k carried at its upper end to engage the ends of the rods.

The construction of the connecting magnet Z of the selector arm F is, in general, similar to that of the coimcctiug magnet of the finder arm E. It comprises the tubular magnet consisting of an iron shell Z with a core Z, about which is placed the winding l The armature Z adapted for a considerable range of movement, is carried upon the ends of the side rods Z, which are supported so as to slide in the enlarged heads of the magnet shell Z are encircled by the compression springs Z to keep the armature normally in its retracted position, and carry upon their outer ends the insulating block 17 upon which the contact springs of the arm are mounted. The contact springs of the lower or finder arm comprise the line springs f each consisting of two metallic strips with their ends bent outwardly to form a fork for grasping the terminal rod adapted to be engaged, and a busy test spring 7, the outer end of which is bent downwardly to lie in a lower plane than the ends of the line springs, and is adapted to make contact with the associated busy tone ring (Z of any of the groups of terminals when the group is rotated upon its axis to bring the busy tone ring forward. The rear end of the busy test spring 2" provided with an upwardly extending portion 7' which, when the armature of the magnet and the associated insulating block Z are retracted, lies in contact with the end 7 of the middle one of the contact rods Z", and is adapted to be separated. therefrom when the insulating block with its associated springs is thrust forward by the attraction of the armature. One of the outer ones of the contact rods Z" carries a contact part which is adapted to be engaged by the contact spring 6 extending from the insulating block carried by the upper arm.

The two arms E, F together constitute a single connecting device, and in order to insure the continuity of the connecting ci"- cuits in all the positions of the arms, a commutator m is provided through. which the circuits between the arms are extended. This commutator comprises a number of concentric rings m m", or separated by insulating material, and upon which the brushes m g m and m respectively, hear. The commutator rings are secured to the hub of the lower or selector arm F, while the brushes are carried upon the upper or finder arm E. The brushes may be, as shown, in the form of curved springs, secured at their centers to connecting or termi nal pieces, and with their ends bearing upon the contact rings. This constructi m is illustrated in Fig. 14,, where the springs m and m are shown attached to the terminal pieces m m respectively, the terminal pieces being insulated from the hub upon which they are mounted.

Surrounding the outermost of the three commutator rings mentioned above, is a toothed ring m having as many teeth as there are groups of line terminals in the automatic switch with which the arms are associated. Cooperating with the toothed ring on is a lever m carried upon the hub of the upper arm, one end of which engages the teeth of the ring, and the other end of which is in position to operate a normally closed switch m This arrangement constitutes a circuit interrupter which becomes operative whenever the relative position of the two arms is changed, as the relative movement of the toothed wheel and the pawl portion of the lever on" which takes place at such a time causes the teeth as they pass beneath the end of the lever to raise and lower it, and thus to actuate the lever to make and break the circuit through the switch m- Therefore, the revolution oi the lower arm while the upper arm is stationary, or viceversa, results in a series (TE interruptions of the current passing through the circuit in which the switch m is included, the number of interruptions depending upon the extent of the relative movement of the arms, and the periodicity depending upon the rate of speed at which the arms move with rela tion to each other.

Referring now to Fig. 16, I shall describe the circuit connections of a pair of arms E, F, which together form a connecting device by means of which communication may be established between any two lines terminating upon the automatic switch. The connecting circuit proper comprises the conductors 5 and (3, the former of which extends from the line contact spring 6 of the finder arm E to the line contact spring f of the selector arm F byway oi the commutator spring m and the commutator ring m and the latter of which extends from the contact spring 6 of the finder arm E to the contact spring f of the selector arm F, by way of the commutator spring m and the commutator ring m. The windings of the magnets 7 and h of the finder arm E are connected in a bridge conductor 9 extending from the conductor 5 at a point between the contact 0 and the commutator spring on by way of the circuit interrupter m The connection of this conductor with the tree pole of the battery 79 is made by way of the shaft 4 with which the inner end of the winding of the magnet 70 is connected, the shaft being in continual connection with the free pole of the battery by way of suitable commutator connections. The connection of the battery is restricted to the shaft 1' and the associated arms by means of the insulating portions interposed between the shaft and the bearings r, r, as mentioned heretofore, which prevent the flow of current from the battery to other parts of the framework. In order to avoid confusion in the connections the battery is not shown connected with the shai't r in Fig. 16; but it will be understood that it is connected in the same manner as illustrated in 15.

The windings of the magnets 7.? and Z of the selector arm F are included serially in a conductor 10 which extends from the connecting circuit conductor 6 at a. point between the contact spring 7 and the commutator ring m to earth or the grounded pole of the battery p by way of the commutator ring on, the commutator spring 0727, the conduct-or 11 and the ground spring 0 of the finder arm E, which is adapted to make connection with the ground ring cl of an associated group of line terminals. A conductor 14 extends from the conductor 10 at a point between the windings of the magnets 70 and Z to the contact point with which the busy test spring f engages when the armature of the magnet Z is in its retracted position. The circuit of the conductor 14 is adapted to be c nnpleted to earth, thus shunting the current from the winding of the magnet Z, by the engagement of the busy ring al or a group of terminals with the busy test spring f the circuit thereupon being completed to earth through the cv'mductor 15 xtending through the low resistance winding of an apparatus for applying the busy tone to the line. Another branch from the conductor 10, which includes the windings of the magnets 7.: and Z, extends by way of conductor 13 through the normally open contacts (mntrolled by the armature Z" of the magnet in and the normally closed cont-ac of the switch a, controlled by the otl or armati'ire 7.7 of the magnet 2 to the tree pole of the battery by way of the shaft r In like manner a branch circuit extends by way of conductor 12 from the connecting circuit conductor 5 to the earth connection made by the engagement of the ground spring a with the ground ring (Z by way of the normally open contact controlled by the armature h of the magnet h, the normally closed contacts of the switch it controlled by the other armature 7&2 of the magnet 7L, and the conductor 11.

Another branch circuit extends from the connecting circuit conductor 5 at a point between the contact spring f and the commutator ring 122?, by way of conductor 8 and the cooperating contacts 6", of the upper and lower arms to the wiping or test spring a of the upper or finder arm E, the circuit being adapted to be completed by the contac of the test spring 0 with the advanced tor- .ininal rod of a calling line.

Referring now more particularly to Fig. 15 of the drawing, will explain the general circuit arrangement and orga ization of the entire system. 15 illustr oes diagrammatically two telephone lines extending from the substations A and B equipped with the special substation apparatus heretofore described, to a. central ofiice C where they are adapted to be connected by means of the automatic connecting devices described in the foregoing. The substation A is connected with the central ollice by means of the conductors 1, which extend to a group of terminals designated D and terminate upon the terminal rods (Z (Z respectively, of the group. The substation B is connected with the central. ofiice by means of the two conductors 3, 1 which extend to a group of terminals designated D and terminate upon the terminal rods (Z (Z of the group.

The substation equipment at both stations is the same, and need therefore only be described in connection with one of the stations. The equipment comprises, in the present instance, the usual telephone transmitter, telephone receiver, induction coil and condenser connected in a circuit controlled by the usual. telephone switch hook. In addition to the foregoing apparatus the circuit controlled by the switch hook includes one winding 0 of a relay 0, another winding of which is included in a conductor 18 which forms a permanently closed shunt of the contacts of the switch hoolc The windin 0 of the relay 0, being serially included in the talking circuit of the line, is made of low resistance, while the winding 0 of the relay 0, being in shunt of the switch hook contacts, and consequently serving to permanently close the circuit of the line about the opening controlled by the switch hook, is made of high resistance.

The armature of the relay 0 controls, in its switch contact 0 the connection with earth of two conductors 19 and 17, the former of which includes a resistance coil 0* and extends from a normally open contact of the switch hook closed to form a branch to earth from the line when the hook is in its elevated position, and the latter of which extends from the limb 1 of the line and includes the energizing coils of the telephone hell or ringer Z). The telephone hell 7) is, as illustrated, of the vibrating type, and. adapted to be energized by steady or continuous currents.

Associated with the line circuit, but normally disconnected therefrom, is the substation interrupter apparatus a. The circuit of the line extends through normally closed contacts of the switch (1, while the interrupter circuit extends by way of the conductor 20 from the norn'nilly open contact of the switch a through the energizing coils m of the locking magnet of the switch (1 the switch contacts a. of the interrupter, and the winding of the interrupter actuating magnet a lVhen the switch a" is a(:-- tuated the through circuit of the line is interrupted, the limb 1 of the line being left open in the contacts of the switch, and the limb 2 being extended by way of the conductor 20 through the switch and interrupter magnets a and a, respectively, to earth. The circuit thus completed, as will be seen, is subject to the control of the interrupter switch a, the manually operated and magnetically locked switch o and the telephone switch hook of the substation.

The lines from the stations A and B, as has heretofore been stated, are connected at the central ofiice with the line terminal rods (Z (Z of the terminal groups D, D respectively, of the automatic switch mechanism. The connections of the lines with the terminal rods of the groups are so made as not to interfere with the oscillation of the groups upon their axes when the finder arms and selector arms make connection therewith.

The connecting circuit controlled by the finder arm E and the selector arm F of any of the connecting devices, as has heretofore been traced, extends by way of the conductors 5 and (3 from the line contact springs 0 c of the finder arm to the line contact springs f of the selector arm, with the actuating magnets g and h of the finder arm E included together with the interrupter switch m serially in av bridge 9 extending from the connecting circuit conductor 5 to the free pole of the central battery 79; while the magnets in and Z of the selector arm F are included serially in a. bridge conductor 10 extending from the connecting circuit conductor 6 to the ground spring 6 and cooperating ground ring (Z Associated with the magnets of both arms are circuits 12 and 13, respectively, controlled in switch contacts of their respective magnets, which close a low resistance local circuit for the flow of current through the magnets at the moment of their actuation. The shunt circuit about the magnet Z to prevent its actuation, and to apply a busy tone through the connecting circuit to the line, is completed by way of the conductor 14 which is controlled through the medium of the busy test spring f and the cooperating busy tone rings 0Z of the groups of terminals. The busy tone rings of each group of terminals are connected to the busy tone rod d, and the busy tone rods of all of the groups of terminals are connected by way of the common conductor 15 through one winding of of an induction coil a to earth. The resistance of the winding 11 is sufliciently low so that when the circuit in shunt of the magnet Z is closed by way of the conductors 14: and 15 and the busy test spring and busy tone ring d the actuation of the magnet Z is prevented. The other winding 71. of the induction coil a is connected in a circuit with a battery p,which may, if desired, be the common battery Which supplies current to the connecting circuits-and a device 0 which, by rapidly interrupting the circuit produced a tone inductively in the winding 11?.

The operation of the system of my invention is as followsi WVe will assume that the subscriber at station A, whose telephone we may say is No. 1, wishes to call the subscriber at station E, whose telephone we will say is No. 25. The subscriber at station A first lifts his telephone receiver from the switch hook. This closes the circuit through the switch hook contacts, and connects the line with earth through the conductor 19, resistance coil 0 and switch contact 0" of the relay 0. The finder arm test springs a of the six pairs of connecting arms associated with the automatic switch R- at the central ofiice, are constantly making a wiping contact with the advance terminals (Z of all of the unengaged groups of terminals; and consequently as soon as a test spring 6 of any of the pairs of connecting arms makes contact with the terminal rod (Z of the group D associated with the station A, a circuit is completed from the free pole of the central battery through the magnets g, h, of the finder arm E to earth at the calling substation, the circuit being traced by way of the conductor 9 including the magnets g and h and the normally closed interrupter switch m to the connecting circuit conductor 5, thence by way of the conductor 8, the cooperating con tacts 7", c of the upper and lower arms, (which are in engagement, the arms lying together in the same plane) the conductor 7, the test spring 6 the limb 2 of the line to the substation A, one of the windings of the substation induction coil, the transmitter, the switch hook contacts, the conductor 19, the resistance coil and the relay contact 0 The current which immediately flows in this circuit causes the magnet h to withdraw its key from the rotating shaft T and thrust its fork into engagement with the ground or pivot rod d of the group of terminals immediately opposite the terminal group of the calling line, thus arresting the movement of the finder arm and anchoring it at the point where it has stopped. At the same moment the energization of the con necting magnet 9 causes it to thrust its contact springs 6 6 against the line terminal rods (Z (Z respectively, of the group D associated with the calling line A, the oscillation of the group upon its longitudinal axis causing the line terminal rod (Z to be thrust forward to meet the contact spring 6 The operation of the magnets g and 72. is rendered more positive and reliable by means of the local circuit 12, which is closed to permit a strong initial current flow to pass through the windings of the magnets by the attraction of the armature h, and is thereafter opened in the contacts h of the magnet by the attraction of the armature h when it is moved to withdraw the key from the shaft, and anchor the arm.

The moment the contact terminals (Z and 6 come into engagement, a circuit is com pleted by which current flows from the free pole of the central battery through the m agnets 9 and it over the limb 2 of the line to the substation, where a portion is diverted, because of the resistance 0 interposed in the branch circuit to earth, through the low resistance winding 0 of the relay 0, serially interposed in the circuit of the line, and

thence over the limb 1 of the line back to the central office, where it passes through the cooperating contacts (P, 6 and flows by way of the conductor 6 of the connecting circuit to the bridging conductor 10, and through the magnets 70 and Z of the selector arm to earth by way of the ground spring (2 and the ground ring (Z The effect of this current flow through the winding of the relay 0 is to cause it to open the switch contact 0', so that the branch from the line to earth by way of the conductor 19 is interrupted, and the circuit over which the conversation takes place thus becomes a metallic circuit free from earth connections. The current which flows in the metallic circuit of the line passes through the movement controlling magnet Z". of the selector arm F, and first causes the attraction of the armature 71: which, in closing its contact, completes a low resistance local circuit through the magnet from the free pole of the central battery in shunt of the line circuit by way of the conductor 13. The maximum energization effected by the closure of this low resistance local circuit, causes the magnet 70 strongly to attract its armature, and, in so doing, to withdraw the associated key from the rotating shaft and thrust the associated fork 7c into engagement with the ground or pivot rod of the group of terminals opposite the group associated with the calling line, thus stopping the movement of the lower or selector arm. The magnet is, in attracting its key and fork-controlling armature, also interrupts the low resistance energizing circuit 18 in the switch contacts is controlled by this armature, thus leaving this magnet under the control of the circuit extending by way of the line to the substation A.

The connecting magnet Z of the selector arm F would also be energized by the current that flows in the metallic circuit of the line, were it not for the existence of the low resistance shunt about its winding completed over conductor 14 to earth by way of the busy test spring 7, the busytone ring (Z the common conductor 15, and the low resistance winding a of the busy test induction coil or transformer n. The connecting magnet Z, under these conditions, is insuiliciently energized to attract its armature, and therefore the contact springs f f F, are not brought into contact with the line terminal rods CF, (1

It must be remembered that at this stage of the operation the arms of the pair have not separated, and lie with their connecting springs opposite the same group of terminals,namely, the group associated with the calling line A. The connecting magnet of the upper finder arm has been energized, as already explained, to thrust its contact springs into engagement with the terminal rods of the group D; while the operation of the connecting magnet Z of the lower or selector arm F has been prevented by the closing of the shunt circuit 14 and 15 about the magnet by the thrusting forward and into contact with the busy test spring 7' of the busy-tone ring (Z which results from the rotation upon its longitudinal axis of the group of terminals D. p

The sequence of the actions thus far described is as follows. First, the magnets of the upper or finder arm are energized, for the energization of the magnets of the lower or selector arm depends upon engagement between the contact springs 0 c of the upper arm and the line terminals of the calling line; second, the armatures of the magnets g and h of the upper arm are attracted, causing the contact springs 6 e of the upper arm to engage and turn the group of terminal rods of the calling line,-a movement that thrusts the busy tone ring (Z of that line into contact with the busy test spring f of the lower arm; third, current passes over the metallic circuit of the line and through the movement controlling magnet is of the lower arm, causing it to stop the movement of that arm, the current being diverted from the connecting magnet Z by the shunt circuit established by the engagement of the busy tone ring (Z with the busy test contact f Therefore, theselectcr arm operates precisely as if it were testing a busy line, as will be explained hereinafter. The advantage at this stage of the operation is that the contact springs 7, f are not allowed to touch the line terminal rods (Z (Z of the group D which are in engagement with the contact springs 0 e of the upper or finder arm; if they were allowed to come in contact the cross connection of the contact springs f f", of the connecting circuit with respect to cont-act springs 6 6 would connect all four magnets of the connecting arms in a closed bridge of the central battery, and would thus lock both arms to the calling line.

At this stage of the operation both arms of the pair have been disengaged from the rotating shaft and anchored opposite the group of terminals of the calling line, the contact springs of the upper or finder arm being in engagement with the line terminal rods, (Z (P. The subscriber now revolves the pointer arm a of his interrupter until it is opposite the number representing the desired line,in this case number 25, the line extending to the substation B. This rotates the hub associated with the pointer arm and causes the spring of the switch a to ride up out of the depression in the hub, and close the switch contacts. The subscriber then depresses the push button a which controls the switch a. This operates the switch to interrupt the metallic circuit of the line, and to connect the limb 2 of the line to earth by way of the conductor 20, including the clutch magnet a of the switch a", the interrupter switch a, and the energizing magnet a of the interrupter. The establishment of this new circuit arrangement causes the current which had heretofore flowed from the battery through the magnets g and it of the finder arm, to the substation, and back to the central otlice and through the magnet is of the selector arm by way of limb 1 of the line, to be diverted to earth at the substation through the conductor 20 with its associated apparatus. is that the magnets g and it of the finder arm remain energized, and retain that arm in contact with the termina-is of the calling line, while the current flowing through the clutch magnet 64 of the switch 64 causes the switch to remain in its operated position, thus diverting the line current from the magnet 70 of the selector arm. The resultant de energization of the magnet it causes the withdrawal ofthe anchoring fork controlled by the magnet, the ree'ngagement of the key with the rotating shaft 14 and a resumption of the revolution of the lower or selector arm F. The interrupter switch m is so arranged that, as soon as the lower arm begins to move away from the upper arm, the line circuit is periodically interrupted, one break occurring for each group of terminal rods that the selector arm passes. Therefore, in progressing to the terminals of line N o. 25 from line N o. 1, the line circuitwill be opened The result and closed in the contacts of the interrupter switch m twenty-four timesthe number of intervals between 1 and 25and the actuating magnet 64. of the substation interrupter a will be deenergized and energized a like number of times. The action of the armatures of all the magnets in this circuit is sluggish, excepting that of the magnet a of the interrupter, which is quick acting; and the interruptions of the curre: 2 llowing in the line produced by the action of the interrupter switch m are too rapid to affect any of the armatures excepting that of the magnet o Therefore, the latter armature alone moves and, in so doing, causes the pointer arm a to return in a contra-clockwise direction to its normal or home position, where it points to No. 1 on the dial, the return being effected by means of the reciprocating action of the escapement lerer a on the star wheel (N, as lrlctofore de scribed.

Consequently for every step which the se lector arm F advances with relation to the finder arm E, the pointer arm of the substation interrupter 64 returns one step, so that when the selector arm has advanced to the group of terminals associated with line No. 25, which is being called, the pointer arm of the substation interrupter will have returned to its normal or home position, and the cooperating member of the interrupter switch 0. will have fallen into the depression in the hub a of the interrupter. This causes the interrupter switch (1- to open the circuit which has heretofore existed through it and the magnets o and a from the limb 2 of the line. The interruption of the circuit through the clutch or looking magnet e of the switch a" causes the switch to re turn to its normal position, and the consequent closure of the metallic circuit of the line causes current to flow back over the limb l and through the movement controlling magnet is of the selector arm F. The magnet 70 thereupon disengages the arm from the rotating shaft r and locks it in position with its contact springs opposite the terminals of the group upon which the line, Fe. 25, extending to substation B, terminates. it will thus be seen that the devices at the central oiiice and at the substation exercise a reciprocal control over each other, the manually operated interrupter mechanism at the substation bringing about the relative movement of the two connecting arms, this relative movement controlling the return morement of the interrupter mechanism, and the return of the interrupter mechanism limiting the advance movement of the selector arm.

If the called line, No. 25, is engaged, its group of terminals will be oscillated upon its axis to thrust the busy tone ring a forward so that when the selector arm F stops opposite said group of terminals its busy test spring will be in contact with the busy tone ring (Z and the shunting circuit 14:, 153 about the connecting magnet of the arm will consequently be completed, so that the arm will be prevented from thrusting its contact springs into engagement with the line terminal rods of the group. Iiioreover, the engagement of the contact spring 7' with the busy tone ring Z will cause the tone which is generated in the secondary winding 01 of the busy tone transformer a to be transmitted by way of conductors l5 and ll to the bridging conductor 10, whence it portion will flow to the connecting conductor (3 of the pair of arms and out over the circuit of the calling line to substation A, where it will be heard by the calling subscriber. This tone will indicate to the calling subscriber that the desired line is busy, and he will thereupon replace his telephone receiver upon its hook.

If the called line, No. is not engaged, the busy tone ring (Z will be in its retired position, and consequently the shunt circuit by way of conductor let is not established around the connecting magnet 1 of the selector arm when the arm comes to rest. lhe magnet 1 thereupon is operated by the current which flows over the circuit of the calling line 1, 2 to thrust its contact springs into engagement with the line terminals of the group upon which line No. 25 terminates. This causes the busy tone ring (Z to advance and make contact with the busy test spring i but this does not complete the shunt circuit of the connecting magnet 1, because the magnet, in thrusting forward its contact springs, has caused the forwardly extending tongue f of the test spring f to break contact with the pin f against which it normally rests. In this way while the selector arm is prevented from making contact with group of terminals made busy by enga ement therewith of some other selector arm, its actuation is not preven ed by the change in condition of the group of terminals when brought about by its own operation the connecting magnet 1, as indicated ab ac, operates to thrust its contact springs into engagement with the terminals (Z (Z of the line. It will be observed that the connection of the contact springs f and f of the selector arm with the central battery is such that the free pole of the battery is applied, when the selector arm makes connection with a line, to the opposite limb of the line from that to which it is applied when the finder arm connects with the calling line. A a result current flows from the free pole of the central battery by way of the conductor 5 of the connecting circuit to conductor 3 of the called line, and thence through the magnetizing coils of the bell Z) at the substation B to earth by way of the normally closed contact of the relay 0. This bell, as has heretofore been explained, is of the vibrating type which is adapted to be actuated by steady or direct current, and its armature is therefore operated to ring the bell until the called subscriber takes the telephone receiver from its hook. ll hen this is done the cur ent flows through the low resistance winding 0 of the relay (2 at the called substation, interposed serially in the circuit completed by the closure of the contacts of the switch hook, and actuates the relay to open the swi ch contacts controlled by its iii-mature. This interrupts the circuit to earth through the magnet windings of the bell, and stops the ringing. To prevent the bell from recommencing its ringing in case the called subscriber at station 13 Should replace his receiver upon its hook while the connection with the line is still established at the central olhce, the r lay 0 is provided with a high resistance winding 0 which, while insufficient in its power to cause the actuation of the relay switch contacts in the first place, is suflicient to maintain them in their operative position when once they have been actuated by the flow of current through the low resistance winding 0 of the relay.

At the conclusion of the conversation the subscribers at the stations A and B replace their telephone receivers upon the switch hooks. The circuit over the lines through the finder arm magnets g and 7t and the selector arm magnets 76 and Z is thus interrupted, and these magnets deenergized. The deenergization of the movement controlling magnets Z: and it causes the keys 0 and its, respectively, to press against their respective collars ready to engage the slots in the latter, when, in the course of their rotation, they pass beneath the waiting keys. Pending this reengagement the arms are held stationary by the engagement of the forks 70 and 71- respectively, with the pivot or ground rods of the groups of terminals opposite those of the lines which the connecting magnets Z and 9 have just permitted their contact springs to release. hen the slots pass beneath the waiting keys, the keys fall into place, and the forks are simultaneously withdrawn from the pivot rods of the associated terminal groups, the key and the fork of each magnet being, by the construction employed, adapted to move together.

The two arms are now re-united, and ready to be appropriated by any other calling line. Until they are re-united, however, the separation of the contact spring 0 carried by the finder arm, from the contact pin 7, carried by the selector arm, interrupts the circuit to the test spring 6 of the finder arm, so that the contacting of the test spring with the advanced terminal (Z of any line that may be calling prior to the reuniting of the two arms of the pair is ineffective to energize the magnets of the finder arm to cause it to stop and complete connection with the calling line.

1 have illustrated the automatic switchboard as being designed to accommodate fifty lines; but it is apparent that the mechanism may easily be constructed to accommodate as many or as few lines as it may be desired to serve. And where interconnect ing means are to be provided for more lines than may conveniently be accommodated upon a single auton'iatic switchl'ioard such as that illustrated. and described, a number of such switchboards or units may be employed, and caused to cooperate by means of interconnecting trunks in a. manner familiar" to those skilled in the art.

It will be apparent that various features which have been described in their application to the particular system shown, will. be capable of application to other systems of widely different character. For example,

I mention the feature of controlling the moven'ient of a selector or other traveler by means of a controller located at a distant point and arranged to be operated by revertive current impulse transmitted back to such controller by a switch or interrupter operated periodically by the selector or traveler in its advance, the controller being adapted at the end of a predetermined movement, variable at will, to operate a switch which will cause the selector or traveler to be stopped. In the particular apparatus illustrated the stopping of the selector or traveler F is determined by the excitation of the clutch controlling magnet K: which is caused to throw off the clutchwhen said magnet receives current over the bridge circuit reestablished by the switch a" at the end of a predetermined movement of the controller a.

This feature of the invention may be more easily understood apart from its specific embodiment in the system shown in Fig. 15, by reference to Fig. 17, wherein the elements concerned in the selecting movement of the arm F are shown without reference to the other parts of the system. Fig. 17 shows a portion of the selector arm F which is adapted to be advanced by the continuously operating motor mechanism represented by the shaft 9, under control of the clutch operating magnet 70. When the magnet 73 is excited, as by current flowing from the battery 7), over the metallic circuit of the line closed by the telephone switch at the substation and extending back through the magnet k, to earth at the selector, the clutch key is withdrawn from engagement with the rotating shaft 13, and the arm F is stationary. The advance of the rotary arm F to any desired extent, as for the purpose of selection, is determined by the controller in 

